PROJECT SUMMARY. A comprehensive understanding of host factors driving the formation and maintenance of the latent HIV reservoir during suppressive antiretroviral therapy (ART) is imperative for developing cure strategies. We have been taking advantage of work in the emerging field of glycomics (the discipline of defining the structures and functional roles of glycans in biological systems) to understand how glycosylation of circulating (plasma) glycoproteins, including antibodies (IgGs), contributes to HIV persistence during ART. Accumulating evidence suggests that glycan profiles mediate and drive several immunological functions, e.g. higher antibody- dependent cell-mediated cytotoxicity (ADCC), antibody-dependent cellular phagocytosis (ADCP), and anti- inflammatory activities. In our recent publication, we identified certain host glycomic traits that inform chronic inflammation, immune activation, and levels of nucleic acid-based measures of HIV reservoir in vivo: (1) Levels of certain mono- and di-galactosylated non-fucosylated glycomic traits negatively correlate with levels of CD4+ T cell-associated total HIV DNA and RNA during suppressive ART; these particular traits have been associated with higher ADCC and ADCP activities. (2) Levels of sialylated (anti-inflammatory) glycans negatively associate with HIV infection, irrespective of long-term suppressive ART, and are associated with higher levels of CD4+ T cells and lower levels of immune inflammation/activation during suppressive ART. Our findings provide the first proof-of-concept evidence that glycomic alterations known to be linked to different states of inflammation and immune functionality are also associated with several immunologic and virologic measures of HIV persistence. Together, these data allow us to hypothesize that levels of certain circulating glycomic signatures (in particular, galactosylated and sialylated glycomic traits) reflect differential states of viral control, which relate to HIV reservoir size during ART and viral rebound upon ART cessation. We will be taking advantage of banked samples from HIV+ individuals on suppressive ART, who completed an Analytical Treatment Interruption (ATI) study, with documented data on time-to-rebound and viral setpoint upon the cessation of ART. Glycomic profiles will be obtained using advanced glycomic technologies and used in both Aims. In Aim 1: we will determine if levels of sialylated and galactosylated antibody and plasma glycans are linked to the size of HIV reservoir (estimated as post-ATI time-to-rebound and viral load set points). In Aim 2: we will define whether the observed antibody galactosylation profiles are simply biomarkers of disease state or whether they also contribute to viral control during ART and/or upon ART cessation, by influencing plasma-mediated innate immune effector ADCC and ADCP activities. We will examine 1) the relationship between antibody glycosylation and antibody-mediated ADCC and ADCP activities, and 2) the link between this relationship and post-ATI viral rebound and viral set points. We will be taking advantage of the recent advances in the emerging field of glycomics to identify novel host glycomic determinants of HIV persistence in vivo.